#include    <stdio.h>
#include<stdlib.h>
#include    <unistd.h>

#define oops(m,x)   //当linux系统执行代码遇到问题时，就会报告错误
{ perror(m); exit(x); }

int main(int ac, char **av)
{
    int thepipe[2], newfd,pid;              

    if ( ac != 3 ){//输入的命令长度不等于3
        fprintf(stderr, "usage: pipe cmd1 cmd2\n");
        exit(1);
    }
    if ( pipe( thepipe ) == -1 )    //以下是各种错误   
        oops("Cannot get a pipe", 1);

    if ( (pid = fork()) == -1 )         
        oops("Cannot fork", 2);

    if ( pid > 0 ){         
        close(thepipe[1]);  

        if ( dup2(thepipe[0], 0) == -1 )
            oops("could not redirect stdin",3);

        close(thepipe[0]);  
        execlp( av[2], av[2], NULL);
        oops(av[2], 4);
    }

    close(thepipe[0]);      

    if ( dup2(thepipe[1], 1) == -1 )
        oops("could not redirect stdout", 4);

    close(thepipe[1]);      
    execlp( av[1], av[1], NULL);
    oops(av[1], 5);
}